The mind is like a garden. You need to look after it. You need to maintain it, keep an eye on it and feed it. You also need to remove the weeds.
The weeds are all those negative thoughts going 'round your mind. Treat it like a garden and begin the weeding.
As you see negative thought arise, accept it and don't judge it but let it go. You can also replace it with a positive thought, affirmation or visualisation.
Remember that you reap what your sow.
Peace and Love